In a conventional halogen bulb, approximately less than 20 percent of the energy output is radiated as visible light. The remaining approximately greater than 80 percent is radiated as infrared light. Since the purpose of a halogen bulb in a vehicle headlamp application is to illuminate the roadway, the infrared radiation is substantially wasted energy. Hence, such bulb designs are generally considered to be relatively inefficient light providers.
Reflective infrared (hereafter RIR) bulbs are known which employ multi-layer dichroic coatings on the outer surface of the bulb. The dichroic coatings generally are adapted to pass wavelengths of visible light and to reflect back wavelengths of infrared light. The structure of the filament and bulb enclosure is such that reflected infrared light is relatively well focused upon the filament along substantially its entire length. Such arrangement provides for filament heating resulting in a more efficient bulb. This all translates into more visible light output for a given power consumption or less power consumption at a given light output. Such designs may be acceptable for relatively long filaments since end losses are a relatively small fraction of the total infrared radiation.
However, RIR bulbs have been successfully adapted for use in automotive headlamps which are characterized by packaging constraints which favor shorter filament lengths and hence increase the fraction of infrared radiation end losses. Such adaptations generally may be characterized by quasi-elliptical envelope structures which tend to redirect otherwise wasted end radiated infrared energy back to the filament thus reducing the overall fraction of infrared radiation end losses. However, they are limited in their application to vehicles having separate high and low beam reflectors. This application limitation is due to several factors. Combined single reflector high and low beam arrangements require a pair of filaments--one for the high beam and one for the low beam. Inclusion of both filaments in a single compact bulb enclosure, such as for example well known industry standard 9004 transverse or 9007 axial, requires non-axial adjacency of the filaments and hence one or neither, but never both, filaments can be accommodated on a centerline focal point of the bulb enclosure in order to benefit from the infrared redirection. Inclusion of both filaments in a single bulb enclosure is known in the industry standard H4 bulb which is characterized by axial adjacency of the high and low beam filaments. However, an H4 type of arrangement would produce undesirably high end losses. Additionally, the low beam shield which is proximate the low beam filament and internal to the bulb enclosure may be heated to unacceptably high temperatures resulting in shield glow which detrimentally affects the optics.